1. Priority Claim
This application claims the benefit of priority from European Patent Application No. 05003701.9, filed Feb. 21, 2005, which is incorporated by reference.
2. Technical Field
The present invention relates to digital signal processing. In particular, the invention relates to a system for improving the subjective quality of an image that has been digitized.
3. Related Art
Due to their relatively low cost, liquid crystal displays (LCD) are used in a wide range of applications. LCDs can be found in personal computers, personal digital assistants, television sets, cellular phones, vending machines, camcorders, digital cameras, automobiles and many other appliances. In some cases the images displayed on LCDs have an analog origin or have been subject to an intermediate analog transmission.
FIG. 1 shows a structure related to displaying video images on a display device such as an LCD. A display apparatus 100 comprises an analog-to-digital converter 102, an image processor 106 and a display 110. An analog video signal is applied to analog video input 101. The analog video signal could be a black and white composite signal, a color composite signal, separate red, green and blue (RGB) signals, or chrominance and luminance signals. The contents of the analog video signals are moving full-video, but in other cases might be still pictures such as photographs, icons or maps.
An A/D converter 102 digitizes the analog video signal with a certain space and amplitude resolution. In case of a composite video signal the A/D converter also separates synchronization signals and synchronizes to them. The A/D converter puts out a digital value for each pixel within an image that is to be displayed. In FIG. 1, each digital value is 24 bits long. Eight bits are dedicated for each of the intensities of the colors red, green and blue. The three different intensity values are symbolized by the three separate connection lines 103, 104, 105. The connection lines 103, 104, 105 provide the pixel values to the image processor 106. The connections shown in FIG. 1 include eight lines per color. This suggests a parallel data connection.
The image processor 106 may be dedicated hardware assembled on a printed circuit board, a dedicated integrated circuit, appropriate software running on a processor such as a digital signal processor or configurable integrated logic circuits. A Field Programmable Gate Array (FPGA) may be employed as the image processor 106. FPGAs may include several function elements including programmable input/output blocks, memory blocks and configurable logic blocks. These functional elements are interconnected by routing channels. An FPGA may be customized by loading configuration data into internal static memory cells. Values stored in such cells determine logic functions and interconnections within the FPGA. Configuration data can be read from an external serial PROM or written into the FPGA from an external master, such as a controller.
In the master mode, the FPGA reads configuration data from an external computer-readable medium such as a mask programmed memory or other non-volatile solid state memory. In the slave mode, an external controller reads configuration data from any computer-readable medium such as a magnetic disk, magnetic tape, optical disk, printed bar code or any kind of solid state memory and writes it into the FPGA.
One task of image processor 106 is to interface the A/D converter 102 and the display 110, as they may have different data formats, different frame rates, different spatial resolution or different amplitude resolution. Image processor 106 may also perform other tasks such as switching between a plurality of different inputs, providing a graphical user interface, or adjusting image parameters such as brightness or contrast. In FIG. 1, display 110 has lower intensity resolution than the ouput of the A/D converter 102. In this case the display has sixteen bit intensity resolution, five bits each for the colors red and blue and six bits for the color green.
In a case such as this, image processor 106 must perform a rounding operation. This may be merely the truncation of the two or three least significant bits from the output values output from the A/D converter. This rounding or truncation operation may cause an indesirable deterioration of the perceived quality of the displayed image.
The degradation of perceived image quality resulting from the rounding or truncation operation may be best understood with reference to FIG. 2. FIG. 2 shows an analog video signal 201. The video signal 201 may be an intensity signal of a black and white video, a luminance signal of a color video or an intensity signal of one of the colors red, green or blue. The signal is constant over the entire time period displayed. Such a signal would correspond, for example, to an area in an image having uniform color and intensity. Curve 202 shows the same signal 201 having an interfering periodic signal superimposed thereon. The interfering periodic signal could be, for example, the hum of a power supply, semiconductor drift, irradiation by RF signals, ripple on the power supply from changing load currents, or the like. When the analog signal 202 is digitized by an A/D converter having the thresholds shown with dashed lines 203 to 205, the digitized signal shown in curve 206 results. Although the signal is depicted as continuous lines in curve 206, it is understood that the signal represents time discrete values corresponding to various the pixels within the image. Due to the fine intensity resolution, a step of one LSB is unlikely to be visible to the human eye. However, because of signal the interferance, the digital signal varies with a peak-to-peak amplitude of two least significant bits (LSBs).
At the ordinate, the four least significant bits of the digital values are shown. In this example the two least significant bits will be truncated in order to obtain six bit resolution necessary for the display values from the eight bit values output from the A/D converter. The result of this truncating operation is depicted in curve 207. Due to the lower resolution, the step created by the interferer in the truncated signal 207 is eight times as high as with the original digital signal. As a result it produces regions of perceivably different color or intensity in an area of originally uniform color or intensity.
The impression created to the eye of an observer is illustrated in FIG. 3. Image 310, the original image, has a circular area of uniform grey, rastered into pixels. Image 312 belongs to curve 207. Image 312 exhibits a contiguous area 302 of perceivably darker color. Depending on the frequency of the interferance, the effect on the image may be a contiguous area, horizontal stripes or even complete areas changing the color shading with time from frame to frame or over a plurality of frames.
As the described effects can significantly degrade the quality of the displayed image. Therefore there is a need for a system which reduces the negative effects of reducing the digital resolution of a displayed image.